High temperature stability, interface bonding, and mechanical behavior in. beta. -NiAl and Ni sub 3 Al matrix composites with reinforcements modified by ion beam enhanced deposition

In preparation for experiments with surface modified Al{sub 2}O{sub 3} reinforcements in {beta}NiAl, diffusion bonding experiments were conducted. FP alumina fibers were prepared with ion sputtered surface films (Al{sub 2}O{sub 3}, Al, Ni) and then composited with {beta}NiAl slabs and hot pressed. After 70 thermal cycles, interfacial shear strength was measured. A roughness mechanism is proposed for the observed increased strength of the coated fibers. Creep in Ni{sub 3}Al was studied. 3 figs, 1 tab. (DLC

High temperature stability, interface bonding, and mechanical behavior in [beta]-NiAl and Ni[sub 3]Al matrix composites with reinforcements modified by ion beam enhanced deposition

Diffusion-bonded NiAl-Al[sub 2]O[sub 3] and Ni[sub 3]Al-Al[sub 2]O[sub 3] couples were thermally fatigued at 900 C for 1500 and 3500 cycles. The fiber-matrix interface weakened after 3500 cycles for the Saphikon fibers, while the Altex, PRD-166, and FP fibers showed little, if any, degradation. Diffusion bonding of fibers to Nb matrix is being studied. Coating the fibers slightly increases the tensile strength and has a rule-of-mixtures effect on elastic modulus. Push-out tests on Sumitomo and FP fibers in Ni aluminide matrices were repeated. Al[sub 2]O[sub 3] was evaporated directly from pure oxide rod onto acoustically levitated Si carbide particles, using a down-firing, rod-fed electron beam hearth; superior coatings were subsequently produced using concurrent irradiation with 200-eV argon ion-assist beam. The assist beam produced adherent films with reduced tensile stresses. In diffusion bonding in B-doped Ni[sub 3]Al matrices subjected to compressive bonding at 40 MPa at 1100 C for 1 hr, the diffusion barriers failed to prevent catastrophic particle- matrix reaction, probably because of inadequate film quality. AlN coatings are currently being experimented with, produced by both reactive evaporation and by N[sup +]-ion enhanced deposition. A 3-kW rod-fed electron-beam-heated evaporation source has been brought into operation

Composition of sputtered NiTiX shape-memory and superelastic thin films

Obtaining desired mechanical and transformational properties in shape-memory and superelastic alloys in the NiTiX system (X=Cu, Hf, Pd, Pt, etc.) requires very tight control of alloy composition. While this is not difficult to achieve in melt-solidification, the sputtering process involves a number of mechanisms, such as preferential resputtering, or species-dependent divergence of the sputter flux, which may cause film composition to deviate from that of the sputter cathode. Of particular concern is the tendency for composition to vary with position on the substrate, and to drift over time as the sputter cathode erodes. Neither problem can be addressed by simple adjustment of the cathode composition. In this paper we consider the often-observed tendency for sputtered TiNi films to be deficient in Ti relative to the cathode composition. A preliminary model is presented which simulates the effect of differential angular distribution of the sputter flux between Ti and Ni by adopting a modified cosine law [1] in which the elemental flux is proportional to $\cos\theta/(\rho_{\rm i} \sin^2\theta+\cos^2\theta)$. It is found that different species-dependent values of $\rho_{\rm i}$, for Ni and Ti respectively, have only modest effect on in-plane composition gradients and time-evolution of composition, but that a systematic Ti deficiency is readily produced by setting $\rho_{\rm Ti}<\rho_{\rm Ni}$

Phase-Fraction Evolution During Incomplete Cyclic Transformation in TiNi: Correlation of Analytical Models with Magnetic Susceptibility Measurements

The response of titanium-nickel, and other alloys exhibiting thermoelastic martensite transformations, display phase-coexistence and hysteresis over a wide range of temperature, arising from the fourfold splitting of a single transformation temperature into Mf, Ms, As, and Af. Attempts to precisely characterize the evolution of phase-fraction during temperature reversals that involve incomplete transformation has led to the formulation of various hysteresis descriptions. We examine a description based on the Duhem-Madelung framework, which takes complete transformation behavior as constitutive input, and on this basis formally predicts the hysteresis associated with transformation arrest and reversal. In conjunction with this effort, a series of magnetic susceptibility measurements on TiNi has been conducted using a SQUID susceptometer. The measured hysteresis in the temperature/susceptibility curves are qualitatively described by the Duhem-Madelung model

Shape recovery and stress-induced martensite in TiNi following indentation and wear loading

We present initial findings that are suggestive of the potential for shape-memory and superelastic NiTi alloys to function as useful tribological materials. For example, surface-deformation made with a spherical indenter is found to be almost completely recoverable by the shape-memory effect. A similar form of strain recovely is shown to be possible in thin films and in wear-track profiles. When a martensitic material is subjected to pin-on-disk loading, a substantial fraction of the wear track cross-section can be recovered by heating. It is also shown that the martensite phase can be stress-induced in response to complex loading associated either indentation or pin-on-disk wear tests, indicating that transformational superelasticity may be able to ameliorate wear degradation in the same way that it can limit low-cycle fatigue damage accumulation. Since NiTi can readily be deployed as a sputtered thin film coating, it may be possible to confer these beneficial effects to base metal substrates such as aluminum

Progress on Sputter-Deposited Thermotractive Titanium-Nickel Films

It is now well established that titanium-nickel alloys fabricated as thin films by physical vapor deposition can display the same transformation and shape-memory effects as their ingot-metallurgy counterparts. As such they may find important application to microelectromechanical and biomechanical systems. Furthermore, we show here that titanium-nickel films may be directly processed so as to possess extremely fine austenite grain size and very high strength. These films display classical transformational superelasticity, including high elastic energy storage capacity, the expected dependence of martensite-start temperature on transformation enthalpy, and large, fully recoverable anelastic strains at temperatures above Af. Processing depends on elevated substrate temperatures during deposition, which may be manipulated within a certain range to control both grain size and crystallographic texture. It is also possible to deposit crystalline titanium-nickel films onto polymeric substrates, making them amenable to lithographic patterning into actuator elements that are well-suited to electrical excitation of the martensite reversion transformation. Finally, isothermal annealing of nickel-rich films, under conditions of controlled extrinsic residual stress, leads to topotaxial orientation of Ni4Ti3-type precipitates, and the associated possibility of two-way memory effects. Much work remains to be done, especially with respect to precise control of composition

Irradiation induced phase transformation in undeformed and deformed NiTi shape memory thin films by high energy ion beams

Irradiation Induced Phase Transformation in NiTi Shape Memory Alloy Mr. Thomas Lagrange1; Dr. Robin Schäublin1; Prof. David S Grummon2; Dr. Christian Abromeit3; Dr. Rolf Gotthardt1 1Swiss Federal Institute of Technology Lausanne, IPMC, Lausanne 1015 Switzerland 2Michigan State University, Lansing, Michigan USA 3Hahn Meitner Institute, Berlin Germany In the present work, ion irradiation has been used as a processing technique for modifying the transformation behaviour of a NiTi shape memory alloy. The current study focuses on the relationship between the irradiation damaged microstructure and the modification of the phase transition. NiTi samples were irradiated at room temperature at 5 and 100 MeV with Ni and Au ions, respectively. Irradiation induced microstructure is investigated using transmission electron microscopy. At relatively low ion energy, the implanted region presents an amorphous layer containing precipitates with a B2 structure, which size decreases with increasing dose. The observed nanocrystals may result from increased point defect densities that lower transformation temperatures and stabilize the B2 phase in regions between the displacement cascades. At high energy amorphous tracks are observed. The relative contribution of the electronic and nuclear stopping is discussed together with the formation mechanisms of the irradiation induced phases

Nickel ion irradiation of plasitically deformed martensitic titanium nickel thin films

At present, little is known about the response of the martensitic phase in TiNi alloys to heavy ion irradiation. However, previous studies [2, 3] of ion and electron irradiation show that these alloys are highly susceptible to disorder and amorphization at damage levels below 1 dpa. This substantially affects their transformation characteristics and their shape memory effect. The present study focuses on the effect of 5 MeV Ni ion irradiation of plastically strained ($\varepsilon\sim 4\%$) martensitic TiNi thin films, which is used as a processing technique for a novel out of plane bending actuator. Conceptually, the frustration of the martensitic transformation due to ion beam damage in a 2 $\mu$pm surface layer of a 6 $\mu$pm thick film will create a sharp differential latent strain on reverse transformation. This latent strain causes a two-way bending motion during cycling heating and cooling. This processing technique can be used to do useful mechanical work on both heating and cooling. To better understand the behavior of these ion irradiated thin films, TEM observations and motion experiments were conducted. Results are presented and discussed as they relate to the ion induced microstructure and its influence on the martensitic transformation